1. Field of the Invention
This invention relates to pneumatic radial tires having improved durability in the bead portion.
2. Description of the Related Art
In general, when a pneumatic radial tire is run under loading, a ground contact portion of the tire is subjected to a reaction force from a road surface against the loading, so that a sidewall portion adjacent to the ground contact portion is subjected to deflective deformation, which is transmitted to a bead portion. As a result, compression strain is created in an outer region of the bead portion in an axial direction of the tire, and particularly a large compression strain is created in the vicinity of an outer end of a turnup portion of a carcass ply in a radial direction of the tire. Additionally, the compression strain created by such a deflective deformation promotes a pulling phenomenon of the carcass ply toward a main body side thereof due to inflation under an internal pressure. This in turn produces cracking in rubber surrounding the outer end region of the turnup portion in the radial direction to finally cause separation failure bringing about tire trouble.
In order to control such a compression strain, there has hitherto been proposed a tire as shown in FIG. 12. In the tire of FIG. 12, a stiffener 11 is arranged on a bead core 12 and comprised of a hard stiffener member 15 made from a hard rubber having a substantially triangular shape, which extends substantially outward from the bead core 12 in the radial direction while closely adhering to a main body 14 of a carcass ply 13. A soft stiffener member 16 is made from a soft rubber arranged outward on the hard stiffener member 15 in the radial direction. Thus, the deflective deformation of a sidewall portion 17 creating the compression strain is controlled by the hard stiffener member 15, while the compression strain is mitigated and absorbed by the soft stiffener member 16 to control strain concentration in the outer end region of a turnup portion 18 of the carcass ply 13 in the radial direction. Furthermore, a ply end rubber member 19 made from rubber having a good resistance to cracking growth is arranged in the outer end region of the turnup portion 18 in the radial direction, whereby strain is mitigated in the outer end region to control the occurrence of cracking. In this way, attemps to improve the durability in the bead portion have been made.
In this tire, the bead portion durability is improved to a certain extent. Lately, it is demanded to further improve bead portion durability because the aspect ratio of the tire is becoming lower and longer service life of the tire (inclusive of retreading) is being required for the commercial market.
For this end, the inventors have made various studies with respect to tires satisfying the above requirements and confirmed that the cracking produced in the outer end region of the turnup portion 18 in the radial direction is created by the following cause in addition to the compression strain based on the deflective deformation of the sidewall portion 17 as mentioned above. That is, when the tire is run under loading, the tread portion is forcedly pushed inward in the radial direction at the ground contact area of the tire to expandedly deform the sidewall portion 17 at the ground contact area outward in the widthwise direction. In this case, the degree of deformation in the vicinity of stepping-in side end and kicking-out side end is larger than the degree of deformation in the other region of the ground contact area. Hence, the sidewall portion 17 is torsionally deformed and particularly a large torsional deformation is caused in the vicinity of a position of a maximum tire width. Such a torsional deformation is transmitted to the ply end rubber member 19 arranged in the outer end region of the turnup portion 18 of the carcass ply 13 in the radial direction, whereby cracking is created in the ply end rubber member 19.
In order to control the transmission of torsional deformation to the ply end rubber member 19, it has been confirmed that it is effective to mitigate and absorb torsional deformation on the way of transmission with the soft stiffener member 16 by decreasing the hardness of the soft rubber stiffener member 16.
However, when deformation is facilitated by decreasing the hardness of the soft stiffener member 16 as mentioned above, the soft stiffener member 16 sandwiched between the main body 14 and the turnup portion 18 of the carcass ply 13 is crushed in the thickness direction by compression force based on the internal pressure filling in the tire to cause a large flow deformation outward in the radial direction. Hence, the ply end rubber member 19 is deformed following such a flow deformation, whereby the effect inherent to the ply end rubber member 19 for mitigating the strain in the outer end region of the turnup portion 18 in the radial direction can not sufficiently be developed.